This comment examines, in the context of disclosure rules, the proposition that any judgment of the propriety of maintenance of a class action necessarily entails a definition of the elements of a cause of action. If this necessity is not explicitly recognized, the unfortunate consequence may be a failure to consider fully the merits of a change in substantive requirements. In the securities law field, where pressures for the maintenance of class actions are particularly strong, the dangers of ill-considered change of substantive law are acute and counsel careful examination of the implications of expansion of 10b-5 liability.